In recent years, higher integration, higher functionality, and higher density mounting of semiconductors widely used in electronic equipment, communication instruments, personal computers, and the like have been increasingly accelerated, and the demand for the thermal expansion coefficient, drillability, heat resistance, and flame retardancy of laminates for semiconductor plastic packages has increased more than ever before.
Particularly, in recent years, a reduction in the thermal expansion coefficient of the laminate in the planar direction has been strongly required. This is because the difference in thermal expansion coefficient between a semiconductor device and a printed wiring board for a semiconductor plastic package is large, and when a thermal shock is applied, warpage occurs in the semiconductor plastic package due to the difference in thermal expansion coefficient, and poor connection occurs between the semiconductor device and the printed wiring board for a semiconductor plastic package, and between the semiconductor plastic package and the mounted printed wiring board.
In order to decrease the thermal expansion coefficient while satisfying heat resistance and flame retardancy, there is a method of increasing the amount of an inorganic filler filled in a resin composition (for example, see Patent Literatures 1 and 2). However, problems of this method are that the cured product is hard and brittle, the drill bit wears quickly, and the frequency of the replacement of the drill bit increases due to the breakage of the drill bit or a lowering in accuracy of hole position.
As another method, there is a method of filling an inorganic filler having high Mohs hardness. For example, methods of blending boehmite or silica as an inorganic filler are known. Although these methods are effective in decreasing the thermal expansion coefficient, a disadvantage of these methods is that because of the hard inorganic filler, the wear of the drill is severe, and the drillability worsens.
On the other hand, when magnesium hydroxide or aluminum hydroxide is blended as an inorganic filler having lower Mohs hardness than boehmite and silica (for example, see Patent Literatures 3 and 4), the drillability is better than in a case where boehmite or silica is used alone, but the thermal expansion coefficient in the planar direction obtained with this is not satisfactory, and the heat resistance decreases under the influence of crystal water released during heating.